For electromagnetic (EM) waves, polarization is normally used to describe the oscillating orientation of the electric field. For transmit and receive antennas, polarization is used to identify the orientation of the electric field of the EM waves. It has been shown by experiments and measurements that the polarization characteristics of millimeter EM waves (mmWaves), such as those with a carrier frequency at 60 GHz and higher, is significantly different from that of those EM waves with carrier frequencies below 6 GHz. One of the differences is that the mmWaves and even their reflections may remain strongly polarized during the propagation. As a result, the received signal may be strongly discriminated in terms of polarization by the antenna at the receiver and the antenna polarization may have a significant impact on the link budget of any mmWave communication systems.
The link loss due to the polarization characteristics of antennas and a propagation channel may result from sources in two categories. The first category is a polarization mismatch loss (PML) in power that occurs at the receiver's antenna, due to the polarization mismatch between the incident EM wave and the receiving antenna. The PML is dependent on the polarization characteristics of both transmitter (TX) and receiver (RX) antennas and the propagation environment, i.e., line-of-sight (LOS) or non-line-of-sight (NLOS) reflections. In the case of NLOS, it is further dependent on reflection orders, reflecting medium and angles of reflections, i.e., angles of departure (AODs) at TX and angles of arrival (AOAs) at RX.
In addition to the power degradation from PML, the second source or category of loss is the receiver processing loss (RPL) in performance that occurs in the receiver processing function units such as timing and frequency synchronization, channel estimation, channel equalization, demodulation and decoding, etc. The RPL is due to the changes in the channel impulse response (CIR) profile including delay spread and power distribution, which is also caused by the polarization mismatch. Therefore, the RPL is dependent on the whole set of parameters causing PML as well as the factors related to the algorithms and implementations of the receiver.